To maximize the transmission capacity of an optical communication system, a single optical fiber may be used to carry multiple optical signals in what is called a wavelength division multiplexed system (hereinafter a WDM system). The multiple optical signals may be multiplexed to form an aggregate multiplexed signal or WDM signal with each of the multiple signals being modulated on separate wavelengths referred to as channels. Modern WDM systems have a high traffic capacity, for example, a capacity to carry 100 or more channels at 100 gigabits per second (hereinafter Gb/s) per channel, or more.
A WDM optical transmission system may include a relatively long trunk path (e.g., optical fiber) that may be terminated at a transmitting and/or receiving trunk terminal. Some systems, such as long haul-systems, may have a length between terminals of about 6,000 kilometers, or more, to span large bodies of water (e.g. oceans). Cables providing a trunk path may include one or more electrical conductors for delivering power to subsea components such as repeaters, amplifiers, and branching units, in order to maintain nominal signal power over relatively long distances.
Increasingly subsea optical communication systems are limited by the ability to deliver power to components via a trunk cable. Distances of 10,000 to 12,000 km, for example can be reached by some approaches to power distribution, but the transmission capacity is limited by the voltage and current that can be delivered to the subsea elements by the trunk cable. Options to increase optical bandwidth such as Raman amplification, increasing optical pump power, expanding beyond the “C” band and increasing the number of amplified fibers unfortunately require power in excess of present system capabilities. Cable resistance reduction based on increasing the quantity of high conductivity materials introduces a significant increase to the cable cost. System voltage increases also typically increases cost, and are often constrained by production technology and materials. Some gains can be made through careful design and engineering, but constraints remain and the cost of re-architecting an existing undersea optical communication system, e.g., modifying a trunk cable, and its associated functional elements (e.g., branching units, amplifiers, and so on), raise numerous non-trivial challenges.